Composite panels are used in a wide variety of applications where high strength and light weight are required such as in the aircraft industry. A composite panel can be made of a structural core sandwiched by a pair of skins or prepregs. A prepreg is a fabric reinforcement that has been pre-impregnated with a resin system, such as a phenolic resin. The resin system is typically an epoxy that includes a respective curing agent. After the prepreg is cured and bonded to the core, the prepreg is considered a skin or a face sheet. A staging classification describes partial curing of the prepreg thermosetting resin system. An A-stage is commonly referred to as a monomer stage, a B-state intermediate typical of prepregs, and a C-stage fully cured material.
One such composite panel is the A502 panel intended for use in primary and secondary aerospace applications, offered by Teklam Corp. of Corona, Calif. The Teklam A502 panel includes a 2024T3 clad aluminum facings of a thickness of 0.02″ and a one quarter inch 5052-0015-3.4 aluminum honeycomb core. See http://www.teklam.com/teklam_A502_panel.html for data sheet and further information. The A502 panel has an evenly distributed low resin content with an auxiliary film adhesive for bonding the honeycomb core and the aluminum facings.
The A502 panel includes a film and an adhesive that can be represented as follows: a prepreg with 0.02 lbs/sqft (pounds per square foot) of an adhesive resin evenly distributed throughout, a film adhesive weighing 0.03 lbs/sqft, and a honeycomb core.
In a traditional prepreg, the resin content is distributed evenly throughout the prepreg so that an amount of resin on one surface is equal to the amount of resin on the other surface. The overall weight of the fabric reinforcement and the resin in the foregoing example is 0.05 lbs/sqft (0.02 lbs/sqft prepreg and 0.03 lbs/sqft adhesive).
The manufacture of this panel requires cutting and laying up of the two materials onto the core, which adds to its manufacturing cost. The inventor realized that a resin layer is applied across the core in an even distribution despite the fact that it is not necessary to do so.
Typically, an adhesive is used to bond the face sheets to the honeycomb core. The adhesive must rigidly attach the face sheets to the honeycomb core in order for loads to be transmitted from one face sheet to the other. If the adhesive fails, the strength of the panel is severely compromised. The adhesive is especially critical in composite panels that use a honeycomb design for the core because of the relatively small surface area over which edges of the honeycomb core contact the face sheets.
One procedure for applying the face sheets to the honeycomb core involves using a prepreg sheet that includes at least one fibrous reinforcement layer and an uncured resin matrix. As a result, the prepreg can be laid into a mold without addition of additional resin and without additional steps required of a typical hand lay-up.
A film adhesive is typically added to the core and it is then bonded to the honeycomb by curing of both the prepreg resin and adhesive resin at an elevated temperature. The film adhesive can be applied as a separate ply layer or as an integral part of the prepreg sheet. Inclusion of the film adhesive is necessary in the prior art construction and adds additional weight to the panel.
An alternative method of bonding the face sheets to the core involves applying an adhesive to edges of the core. The adhesive is typically applied by dipping the edges of the core in the adhesive. Adhesives used in this type bonding are typically referred to as dip resins or dip adhesives. The advantage of this method is that the adhesive is located only where the edges of the core contact the face sheet, rather than being distributed over the entire face sheet. This method is generally used to bond non-adhesive face sheets, such as aluminum and other metallic face sheets, to the core.
There is a need in the aerospace and transportation industry for an improved panel having reduced weight.